Cache more server responses for faster loading on app-switch/startup

[MrApplejuice]

Problem

When using the app, the history is most useful when it allows a "quick glance" of the past activities. For this to work well, it should load as fast as possible. However, when the app is opened now, it takes a second or two - sometimes even longer - for any items to appear.

This is likely to get worse when more activities are fetched for the list of

Solution

Show an indicator that loading is happening, but complement this by showing a cached version of the list that was loaded when the app was previously launched.

Details

I would like to do this by abstracting and improving the entire ChildObserver-logic. The class has grown overly complicated, but could easily do this b breaking it into abstract submodules. Kind of a lot of work but should pay off big time down the line.